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  • mbenkerumass 6:00 am on February 28, 2020 Permalink | Reply
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    Tape Measure Yagi Antenna 

    Radio Club Meeting – Tape Measure Yagi build

    image3image2image160082311118__8CCDCFDA-BFA7-4E2D-8351-9F4533FC0B00

     
  • mbenkerumass 6:00 am on February 25, 2020 Permalink | Reply
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    Optical Waveguides 

    Just as a metallic strip connects the various components of an electrical integrated circuit, optical waveguides connects components and devices of an optical integrated circuit. However, optical waveguides differ from the flow of current in that the optical waves travel through the a waveguide in a spatial distribution of optical energy, or mode. In contrast to bulk optics, which guide optical waves through air, optical waveguides guide light through dielectric conduits.

    Bulk Optical Circuit:

    waveguide2

    Optical Waveguides:

    waveguide1

    The use of waveguides allows for the creation of optical integrated circuits or photonic integrated circuits (PIC). Take for example, the following optical transmit and receive module:

    optical_transmitrecieve

    Planar Waveguides

    A planar waveguide is a structure that limits mobility in only one direction. If we consider the planar waveguide to be on the x axis, then the waveguide may limit the travel of light between two values on the x axis. In the y and z directions, light may travel infinitely. The planar waveguide does not serve many practical uses, however it’s concept is the basis for other tpyes of waveguides. Planar waveguides are also referred to as slab waveguides.Planar waveguides can be made out of mirrors or using a dielectric with a high refractive index slab. See also, Planar Boundaries, Total Internal Reflection, Beamsplitters.

    waveg1waveg2

    Rectangular Waveguides

    Rectangular waveguides can also be built either from mirrors or using a high refractive index rectangular waveguide.

    waveg3waveg4.png

    The following are useful waveguide geometries:

    waveg5

    Various combinations of waveguides may produce different and useful configurations of waveguides:

    waveg6

     

     

     

     
  • mbenkerumass 6:00 am on February 24, 2020 Permalink | Reply
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    Optoelectronic Integrated Circuit Substrate Materials 

    The substrate material used on an optical integrated circuit (OIC) is dependent primarily on the function performed by the circuit. An optical integrated circuit may consist of sources, modulators, detectors, etc and no one substrate will be optimal for all components, which means that a compromise is needed when building an integrated circuit. There are two main approaches that taken to deciding on a solution to this compromise: hybrid and monolithic approaches.

     

    Hybrid Approach

    The hybrid approach attempts to bond more than one substrate together to obtain an optimization for each device in the integrated circuit. This approach allows for a more optimized design for each component in theory, however the process of bolding the various elements together is prone to misalignment and damage from vibration and thermal expansion. For this reason, although the hybrid approach is a theoretically more otpimized approach, it is more common to use the monolithic approach for OIC.

     

    Monolithic Approach

    The monolithic OIC uses a single substrate for all devices. There is one complication in this approach which is that most OIC will require a light source, which can only be fabricated in optically active materials, such as a semiconductor. Passive materials, such as Quartz and Lithium Niobate are effective as substrates, however an external light source would need to be coupled to the substrate to use it.

     

    Optically Passive and Active Materials

    Optically active materials are capable of light generation. The following are examples of optically passive materials:

    • Quartz
    • Lithium Niobate
    • Lithium Tantalate
    • Tantalum Pentoxide
    • Niobium Pentoxide
    • Silicon
    • Polymers

    The following are optically active materials:

    • Gallium Arsenide
    • Gallium Aluminum Arsenide
    • Gallium Arsenide Phosphide
    • Gallium Indium Arsenide
    • Other III-V and II-VI semiconductors

     

    Losses in Substrate due to Absorption

    Monolithic OICs are generally limited to the active substrates above. Semiconductors emit light at a wavelength corresponding to their bandgap energy. They also absorb light at a wavelength equal to or less than their bandgap wavelength. It follows then, for example, if a light emitter, a waveguide and a detector are all fabricated in a single semiconductor, there is a considerable issue of light being absorbed into the substrate, meaning that not enough light will be present for the detector. Thus, reducing losses due to absorbtion is one of the main concerns in substrate materials.

    substrate

     
  • mbenkerumass 6:00 am on February 22, 2020 Permalink | Reply
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    Gas Laser and Semiconductor Lasers 

    heliumconstruction

    The Gas Laser

    In laboratory settings, gas lasers (shown right) are often used to eveluate waveguides and other interated optical devices. Essentially, an electric charge is pumped through a gas in a tube as shown to produce a laser output. Gasses used will determine the wavelength and efficiency of the laser. Common choices include Helium, Neon, Argon ion, carbon dioxide, carbon monoxide, Excimer, Nitrogen and Hydrogen. The gas laser was first invented in 1960. Although gas lasers are still frequently used in lab setting sfor testing, they are not practical choices to encorperate into optical integrated circuits. The only practical light sources for optical integrated circuits are semiconductor lasers and light-emitting diodes.

     

    The Laser Diode

    ladio

    The p-n junction laser diode is a strong choice for optical integrated circuits and in fiber-optic communications due to it’s small size, high reliability nd ease of construction. The laser diode is made of a p-type epitaxial growth layer on an n-type substrate. Parallel end faces may functions as mirrors to provide the system with optical feedback.

     

    The Tunnel-Injection Laser

    The tunnel-injection laser enjoys many of the best features of the p-n junction laser in it’s size, simplicity and low voltage supply. The tunnel-injection laser however does not make use of a junction and is instead made in a single crystal of uniformly-doped semiconductor material. The hole-electron pairs instead are injected into the semiconductor by tunneling and diffusion. If a p-type semiconductor is used, electrons are injected through the insulator by tunneling and if the semiconductor is n-type, then holes are tunneled through the insulator.

     
  • mbenkerumass 6:00 am on February 20, 2020 Permalink | Reply
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    The Quantum Well 

    What is a Quantum Well

    Optical Integraded devices are normally built with the consideration that the device size will be large compared to the wavelength of the beams in the system. When however, the device size is reduced to a size of the same order of magnitude as the wavelength of light in the system, unique properties can be observed. The class of device that operates under the unique properties of this arrangement is the “quantum well.”

     

    Uses of Quantum Wells

    Quantum wells may be integrated to other optical and opt-electronic integrated circuits. Uses of quantum wells include improved lasers, photodiodes, modulators and switches.

     

    Building a Quantum Well

    A quantum well structure features one or more very thin layers of narrow bandgap semiconductor material, interleaved with layers of wider bandgap semiconductors. The thickness of the layers in a quantum well are typically 100 Angstroms or smaller. Quantum wells with many layers are termed a “Multiple Quantum Well” (MQW) structure and quantum wells with only one layer are termed a “Single Quantum Well (SQW) structure. A typical MQW structure may have around 100 layers. The GaAs-AlAs material system or GaInAsP are common choices for materials in quantum well structures.

    quantumwell1

    Superlattice Structure

    A superlattice structure is a term for a case in whic a multiple quantum well structure is built with barrier wals that are thin enough that electrons are able to tunnel through the structure.

     
  • mbenkerumass 6:00 am on February 15, 2020 Permalink | Reply
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    Hermite-Gaussian, Laguerre-Gaussian and Bessel Beam 

    The Gaussian Beam [link] is not the only available solution to the Helmholtz equation [link]. The Hermite-Gaussian Beam also satisfies the Helmholtz equation and it shares the same wavefronts (shape) of the Gaussian Beam. Where it differs is in the distribution of intensity in the beam. The Hermite-Gaussian Beam distribution is a modulated Gaussian distribution in the x and y directions which can be seen as a number of functions in superposition. The below figures depict the cross-sections of ascending order intensity distributions for the Hermite-Gaussian Beam. Secondly, distribution orders zero through three are shown.

    hermitegau2hermitegau1

    The Complex amplitude of the Hermite-Gaussian beam labeled by indexes l,m (orders):

    hermitegaussian

     

    Laguerre-Gaussian Beams

    The Laguerre-Gaussian Beam is a solution to the Helmholtz equation in cylindrical coordinates.

    laguerregau1

    The shape of the Laguerre-Gaussian Beam intensity distribution is of a toroid which increases in radius for orders where m = 0 and for orders m > 0, it takes the form of multiple rings.

    laguerregau2

     

    The Bessel Beam

    The Bessel Beam, by comparison to the Gaussian Beam differs in that it has a ripple effect by oscillation in addition to a similar gaussian curve. The complex amplitude of the Bessel Beam is an exact solution to the Helmholtz equation, while the complex amplitude of the Gaussian beam is an approximate solution (paraxial solution).

    besselbeam

    B. E. A. Saleh and M. C. Teich, Fundamentals of photonics. Hoboken: Wiley, 2019.

     
  • mbenkerumass 6:00 am on February 13, 2020 Permalink | Reply
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    Gaussian Beam Transmission Through Optical Components 

    The most important note about the transmission of a Gaussian Beam [link] through various optical components [link] is that the beam will remain Gaussian, given that the system is paraxial. The shape of the Gaussian beam will change according to the components, however.

    The complex amplitude of the Gaussian beam (width) is adjusted to the width of an optical component, for example.

    beam8

    The Gaussian beam that emerges from the above lens takes the following formulas:

    beam9

    Lenses may be used to focus the a Gaussian beam. This is achieved by positioning the lense appropriately according to the location of the beam waist. For applications such as laser scanning and compact-disk burning, it is desired to focus the beam to the smalles size possible.

    beam10

    The focused waist W0′ and the distance of the focused waist z’ are a function of the waist of the original beam and the focal length f of the lens.

    beam11

    Beams may also be relayed and expanded using lenses.

    relaylens

    beamexpansion

    A Gaussian beam, as do rays and waves behave differently for a plane mirror (i.e. spherical mirror with infinite radius) and spherical mirrors.

    beam12

    As is the case with geometrical ray optics, beam properties through a system can be modeled using the ABCD matrix method.

    beam13

    B. E. A. Saleh and M. C. Teich, Fundamentals of photonics. Hoboken: Wiley, 2019.

     
  • mbenkerumass 6:00 am on February 12, 2020 Permalink | Reply
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    The Gaussian Beam 

    Wave optics as previously discussed operated under an ideal assumption that light can be confined to a uniform, rectangular shape that moves through space. A more realistic understanding of a wave that propagates through space is the goal of beam optics, which instead describes a light wave as a distribution of light.

    The Gaussian Beam

    The Gaussian beam is a common description of the distribution of a light beam which satisfies the Helmholtz equation. Light is concentrated towards the center of the beam in a Gaussian distribution.

    gaussianbeam

    The width of the beam is a minimum at what is termed the waist of the beam and the width increases at distances further from the waist. Eventually, the width of the beam would become very wide and the distribution of light would be wide enough, almost to approximate a spherical beam. In reference to the figure above, the leftmost distribution may for example be the distribution at the waist of the beam and the rightmost picture is the beam further from the waist. In a localized area, the beam exhibits similar characteristics to the ideal plane wave.

    beam1

    The width of the beam is determined by the following formula:

    beam2

    beam3

    The complex amplitude of the Gaussian beam is described by the following formula:

    beam4

    Further parameters of the beam used in the above formula are the following:

    • W(z): Beam width function (above)
    • R(z): wavefront radius of curvature
      beam5
    • ξ(z): Beam center point
      beam6
    • W0: Minimum Beam level, found at z = 0
      beam7

    B. E. A. Saleh and M. C. Teich, Fundamentals of photonics. Hoboken: Wiley, 2019.

     
  • mbenkerumass 6:00 am on February 10, 2020 Permalink | Reply
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    Wave Optics – Interference, Interferometers 

    Interference
    interference1

    When two or more waves of the same frequency are present in the same location, the sum of their intensities may not equal the intensity of the total wavefunction. The interference is understood as the difference between the intensity of the total wavefunction and the sum of the individual wavefunction intensities. 

    The interference equation is used to talculate the intensity of the total wavefunction. The third term is the interference between the two waves, where φ is equal to the sum of the phases of the two waves.

    interference2

    When adding wavefunctions of different phases, these wavefunctions can be drawn as a superposition of vectors, where the intensity of the wavefunction in the magnitude and the phase is the angle of the wavefunction vector.

    interference3

    Consider the case in which two waves, represented by two vectors are equal in magnitude, but 180 degrees out of phase of each other. In this case, the intensity of the total wavefunction is zero. If there is no phase difference between the two wavefunction vectors, then the interference of the two waves is zero and the maximum intensity of the system is reached.

    Interferometers

    It has been mentioned that Wave Optics and Geometrical Optics are insufficient to take measurements of the intensity of rays and waves. However, by determining the level to which waves interfere with each other, a relative intensity can be measured. The interferometer is an instrument that detects the intensity of the a superposition of waves of a varied phase difference. A wave is split using a beamsplitter and each split wave is reflected after different (or possibly the same) distances and recombined. After recombination of the optical waves, the interference is measured by amount of loss in the system and subsequently the distances of the mirrors. Applications include metrology, measurements of refractive index and spectrometry.

    Three prominent examples of interferometers are the Mich-Zehnder interferometer, the Michelson interferometer and the Sagnac interferometer.

    interferometer1interferometer2

    B. E. A. Saleh and M. C. Teich, Fundamentals of photonics. Hoboken: Wiley, 2019.

     
  • mbenkerumass 7:13 pm on February 7, 2020 Permalink | Reply
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    The Bipolar Transistor, Modes of Operation 

    The transistor is a multifunction semiconductor device that, when used with other circuit elements has the ability to produce a current gain, voltage gain and signal-power gain. The transistor is referred to as a passive device, while the diode is passive. The three basic types of transistor technologies are the bipolar transistor, the metal-oxide-semiconductor field effect transistor (MOSFET) and the junction field effect transistor (JFET). The bipolar transistor most often functions as a voltage-controlled current source.

    The Bipolar Junction Transistor

    The BJT has three separately doped regions and two pn-junctions, which are close enough to interact between each other. The BJT can either be constructed as an NPN or PNP transistor, which stands for the arrangement of positive and negatively doped regions.

    pnpnpn

    The main connections of a BJT transistor are referred to as the collector, base and emitter. Generally, the emitter side is doped to a higher level than the collector. The result of this is that when a supplied a voltage, the electrons will flow in the direction from the emitter to the collector. The direction of current then will be from the collector to the emitter.

    npnpnp2pnp1

    BJT Modes of Operation

    There exist three modes of operation for the BJT transistor. In reference to the diagram below, when the Base-Emitter voltage is zero or reverse biased, the majority of carrier electrons from the emitter will not be injected into the base. This mode where all currents in the transistor are zero is referred to as cut-off. When the Base Emitter voltage is positive (forward biasing), an emitter current is generated. As the Base Emitter voltage increases, the collector current will continue to increase until a certain point at which both the Base Emitter and Base Collector junctions become forward biased. This mode is called saturation.

    npppnn

     
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